An activation domain in the C-terminal subunit of HCF-1 is important for transactivation by VP16 and LZIP

In herpes simplex virus, lytic replication is initiated by the viral transactivator VP16 acting with cellular cofactors Oct-1 and HCF-1. Although this activator complex has been studied in detail, the role of HCF-1 remains elusive. Here, we show that HCF-1 contains an activation domain (HCF-1(AD)) required for maximal transactivation by VP16 and its cellular counterpart LZIP. Expression of the VP16 cofactor p300 augments HCF-1(AD) activity, suggesting a mechanism of synergy. Infection of cells lacking the HCF-1(AD) leads to reduced viral immediate-early gene expression and lowered viral titers. These findings underscore the importance of HCF-1 to herpes simplex virus replication and VP16 transactivation.     

Activator-Mediator binding regulates Mediator-cofactor interactions

The 26-subunit, 1.2 MDa human Mediator complex is essential for expression of perhaps all protein-coding genes. Activator binding triggers major structural shifts within Mediator, suggesting a straightforward means to spatially and temporally regulate Mediator activity. By using mass spectrometry (MudPIT) and other techniques, we have compared the subunit composition of Mediator in three different structural states: bound to the activator SREBP-1a, VP16, or an activator-free state. As expected, consensus Mediator subunits were similarly represented in each sample. However, we identify a set of cofactors that interact specifically with activator-bound but not activator-free Mediator, suggesting activator binding triggers new Mediator-cofactor interactions. Furthermore, MudPIT combined with biochemical assays reveals a nonoverlapping set of coregulatory factors associated with SREBP-Mediator vs. VP16-Mediator. These data define an expanded role for activators in regulating gene expression in humans and suggest that distinct, activator-induced structural shifts regulate Mediator function in gene-specific ways.